Multiporous ceramics are materials widely used as basic materials in various industries, e.g., filters, catalyst carriers, sensors, refractory materials, lightweight structural materials, and preform materials for infiltration. However, when pore size distribution in this type of multiporous ceramics is wide, or the distribution of pores is uneven, the stress applied to the material is not distributed across the entire material but is concentrated to the weakest portion where there is a cluster of pores, which in the end lowers its strength and leads to non-homogeneous strength distribution. Accordingly, with respect to multiporous ceramic materials, the control of pore size and the distribution of pores are important factors for quality enhancement.
In general, the production methods of multiporous ceramic materials can be classified mainly into two methods. The first method involves a process of producing multiporous ceramic materials by adding and then mixing pyrolytic or volatile materials to ceramics, and then forming pores by volatilizing the volatile component in said mixture. This type of methods is exemplified by U.S. Pat. No. 5,358,910. To summarize, the method in U.S. Pat. No. 5,358,910 comprises mixing ceramics and polymers by for example first ball-milling them, making said mixture into molded bodies of a certain shape, and heating said molded bodies so as to carry out pyrolytic reaction by combusting the combustible materials and volatilizing the volatile materials in the polymer component of the molded bodies. Meanwhile, the ceramic component in the molded bodies is sintered by heating, and the volatile materials in said polymer component are volatilized. In the end, in their places, the pores are formed, thereby resulting in production of multiporous ceramic materials.
However, this type of methods involves a necessary process of mixing ceramics to pyrolytic or volatile materials, and in this process of mixing, it is impossible to mix the raw materials with 100% homogeneity. Accordingly, there are disadvantages in that the sizes of the pores formed as such (i.e., in places of volatile materials after volatilization) and the distribution of such pores are not uniform. Moreover, it is rather difficult to control the pore size with the desirable characteristics of the material because of the wide size distribution of the pyrolytic or volatile materials.
As for the second method, it involves producing multiporous ceramic materials by partial sintering of ceramics.
The second method may further be sub-classified into two sub-methods. The first one involves forming more pores by adjusting the sintering conditions of ceramics for example by lowering relative density of ceramics by sintering ceramics at a temperature appropriately lower than the optimal sintering temperature. However, the multiporous ceramics produced by this method are not sintered under the optimal sintering conditions, and as such, the mechanical properties, such as strength, are significantly deteriorated.
Moreover, there is a method of producing multiporous ceramics, such as in the invention as disclosed in U.S. Pat. No. 6,214,078, which involves lowering sinterability by using the differences in particle sizes of ceramic raw materials. The method involves manufacturing molded bodies of a certain shape by mixing ceramic raw materials of small and large particle sizes, respectively, and sintering them by heating. As such, the large particles, which have relatively small surface energy acting as driving force for sintering process as compared to that of the smaller particles, end up hindering the sintering process therein. As a result, the method produces multiporous ceramics by lowering relative density of ceramics to form pores therein. However, this method too includes a mixing process of raw materials and has the same problems of non-uniformity of pore size and distribution, in addition to the ensuing difficulties in controlling pore size, which are all due to non-homogeneous mixing.
Meanwhile, although not an invention relating to ceramics, U.S. Pat. No. 5,158,986 discloses a method of producing microporous plastic materials. The method involves saturating plastic materials using supercritical CO2 and forming a large number of bubbles via a rapid pressure drop. In other words, the method produces microporous plastic materials of relatively even distribution of pores across the entire materials by using supercritical CO2 as a medium for forming pores.
However, as for the method disclosed in U.S. Pat. No. 5,158,986, it has a disadvantage of high cost since the method requires additional equipment, such as a heating device for making supercritical CO2 as a medium for forming pores. Moreover, it also has the problem of complicating operation process since the method requires additional preliminary processes, i.e., such as for making supercritical CO2, the processes of reducing specific volume by cooling saturated CO2, and re-pressurizing or heating.